The fluid nature of the erythrocyte membrane at the molecular level implies the existence of a viscosity at the continuum level. To measure whole membrane viscosity, we have developed direct microforce techniques which allow us to impose known shear forces per unit length in the plane of the membrane (shear resultants) and measure the resulting in-plane rates of deformation. The ratio of the shear resultant to the rate of deformation is the shear modulus of surface viscosity. From these measurements we have identified two surface viscosities: a recovery viscosity which characterizes irreversible (plastic) membrane deformation. We hypothesize that the recovery viscosity characterizes the flow of the lipid-integral protein component of the membrane while the plastic viscosity characterizes the flow of the peripheral protein component (i.e., spectrin). Our objectives in the proposed research are to measure both the recovery viscosity and the plastic viscosity as a function of temperature (0-50 degrees C) and pH and to relate these measurements to altered states of the membrane. Dual micropipette aspiration will be used to elastically (reversibly) deform the cell from opposite ends. The recovery time measured after subsequent release gives a direct measure of recovery viscosity. The rate of plastic growth of cell membrane tethers in a parallel plate flow channel characterizes the plastic viscosity. Data is recorded and analyzed on a TV-microscope system, except at high temperature were high speed photography will be used in the cell recovery experiments. Experiments will be performed in microchambers and flow channels with temperature control over the range 0 degrees - 50 degrees C.